This invention relates to an improved method and means in the processing of raw liquid sewage which contains suspended matter therein.
One typical treatment for raw sewage, consisting of water contaminated by organic and inorganic matter which is dissolved in the matter as well as organic and inorganic matter which is not dissolved but is suspended, is to mix the sewage after screening with activated carbon and then dump the mixture into a primary settling tank, to permit the activated carbon to adsorb the dissolved pollutants and also to assist in settling the suspended matter to the bottom of the tank as sludge. Thereafter, the liquid contents of the primary settling tank are again mixed with the activated carbon and then placed in a second settling tank for further adsorption of dissolved pollutants and settling of suspended matter to the bottom of the tank to form a second sludge. The contents of the second tank may then be filtered to catch any fine suspended matter such as fine charcoal particles, and the water may then be returned to a stream or to the ocean.
The sludge which settles to the bottom of the first and second settling tanks can be treated to be converted into something useful. One technique is to de-water the sludges to form sludge cakes which are then pyrolyzed to form activated carbon and ash. This activated carbon and ash may then be used to supplement the carbon otherwise required to be added.
It has been found that the ash content of the material coming out of the pyrolyzer is quite high, as much as 75%. This consists primarily of sand and clay that comes in with the raw sewage. This excessive ash constitutes a bulk which interferes with the effective adsorption of the dissolved solids and damages the ability of the carbon to meet the Biological Oxygen Demand (BOD) standards that are set forth for the sewage effluent. Also, the mixture of the ash and carbon makes the ash difficult to separate from the carbon. Thus the necessary removal of ash to reduce its amount also results in the removal of a proportionate amount of carbon whereby the yield of carbon is reduced. To supply enough carbon and to keep down the need for supplemental commercial carbon it may be necessary to recycle the material a greater number of times. Unfortunately this results in a higher yield of ash.